Various devices have been proposed for processing of signals at very high frequencies. One such device is a surface acoustic wave (SAW) device which has found widespread application for performing analog signal processing functions, such as filtering, correlation, convolution, and pulse compression operations. Such devices tend to be relatively simple and inexpensive to fabricate and have the advantages of being both passive in nature and extremely compact in structure. Exemplary devices have been described in the literature and a typical device is described, for example, in U.S. Pat. No. 4,101,965, issued on July 18, 1978 to K. Ingebrigtsen et al.
Because of the low velocity of surface acoustic waves along the surface of a crystal substrate, relatively long interaction times can be accommodated on a relatively short substrate. Bandwidths of up to about 200 Megahertz (MHz), or so, can be achieved. Such bandwidth and interaction time characteristics make SAW devices highly useful as signal processing devices. However, the fractional bandwidths of surface acoustic wave devices is fixed at about 20% and, therefore, to achieve wider bandwidths the center frequency has to be increased. But the center frequency of an SAW device has a practical upper limit of about 1 GHz. Thus, as mentioned above, the bandwidth is limited to about 200 MHz. Such upper frequency limit tends to arise because of the increasing dispersion of the acoustic wave as it travels along the surface, which dispersion can often be attributable to surface damage and because of the increasing attenuation of the surface acoustic wave due to the substrate material which attenuation generally tends to increase as the square of the frequency. Further, there are limitations in the processes, normally photolithographic processes, for fabricating the surface gratings and the transducers associated therewith, the dimensions thereof becoming so small at extremely high frequencies that the processes used are not capable of providing the required small dimensions.
In order to overcome the frequency limitations on surface acoustic waves, the use of microwave superconducting strip transmission line devices has been suggested. Such approach is discussed, for example, in the article "Passive Superconducting Microwave Circuits For 2-20 GHz Bandwidth Analog Signal Processing" by J. T. Lynch et al., Proceeding of the MTT Symposium, 1982. Such devices, however, require operation at extremely low temperatures (down to as low as about 4.degree. K.), thus requiring a multi-stage refrigeration system which substantially increases the weight and volume of the overall device and requires power for operating the refrigeration system. Moreover, there is a time delay in achieving signal processing operation after the refrigeration system has been made operative. Furthermore, such approach is relatively costly and, because the fabrication of such superconducting strip lines is extremely difficult in comparison with the fabrication of surface acoustic wave devices, they are even more subject to manufacturing defects. Since superconducting strip transmission lines are utilized with electromagnetic waves, the high velocity of such waves severely reduces the interaction time that can be achieved.
Accordingly, it would be desirable to devise an approach that would achieve the high frequency operation required without the disadvantages of either the surface acoustic wave device approach or the microwave superconducting strip transmission approach, at a reasonable cost and with a reasonable ease of fabrication.